English Translation of PCT/JP00/03709 filed Jun. 7, 2000.
Hydrogen Storage Laminated Material
1. Technical Field
The present invention relates to a hydrogen storage laminated material, and more particularly, relates to a hydrogen storage laminated material having excellent hydrogen storage capability.
2. Background Art
With growing interest in hydrogen energy systems in recent years, research and development of materials of the metallic alloys for hydrogen storage have been actively conducted searching for materials for use as a hydrogen storage and transport medium, or for use in energy conversion, separation and refinement of hydrogen gas, and the like. The most important property as the metallic alloys for hydrogen storage is excellent hydrogen storage capability. In the conventional materials, the atom ratio of stored hydrogen to metal (H/M) is as follows: H/M=1.00 for LaNi5 and CaNi5; H/M=1.33 for Mg2Ni; and H/M=1.50 for ZrV2.
In the case where the hydrogen storage material is a massive (bulk) state, the hydrogen storage material is pulverized as a result of repeated hydrogen absorption-desorption cycles. This pulverization significantly hinders the practical use as a hydrogen storage material. Therefore, attempts have been made to form the hydrogen storage material into a thin film that is less susceptible to pulverization. However, the hydrogen absorption amount is reduced as compared to the massive sample. Moreover, in order to use the hydrogen storage material for the electrode materials of the nickel-hydrogen secondary batteries or the like, development of a material having H/M=1.50 or more as a standard of the hydrogen absorption amount has been expected.
Then, in order to solve these problems, a technology was disclosed in which the hydrogen storage capability is improved by a laminated structure of Ti having an hcp structure (group 4A metal, alloy, compound) and Cr having a bcc structure (group 6A, 7A, 8A metal, alloy, compound) (Japanese Laid-Open Publication No. 9-59001). A material having this laminated structure allows for significant improvement in the hydrogen storage capability.
According to the above-mentioned laminated material, H/M of 1.5 or more can be easily achieved, and under the good conditions, H/M of about 3.0 is also possible. However, the following problems arise when the above-mentioned elements are used:
1) A relatively expensive metal is used. Ti is relatively commonly used, but is restricted in terms of resources, and therefore becomes expensive when used in applications such as batteries. In other words, utilization in large quantities is difficult industrially.
2) The weight is increased. The increased weight is highly disadvantageous for portable use or the like.
It is an object of the present invention to provide a hydrogen storage laminated material having an increased H/M value and capable of achieving reduction in weight and of being mass-produced industrially.
A hydrogen storage laminated material of the present invention has a laminated structure of first and second layers, wherein the first layer is formed from an alloy or compound including an element of a group 2A or 3A or an element of at least one of the groups 2A and 3A, and at least partially includes a body-centered cubic structure, and the second layer is formed from an alloy or compound including an element of one of groups 6A, 7A and 8A or an element of at least one of the groups 6A, 7A and 8A.
The inventors have confirmed for the first time that a laminated structure that is durable as an industrial material can be obtained by laminating a layer having an element of the group 2A, 3A and a layer having an element of the group 6A, 7A, 8A, and that this laminated structure is light in weight and also has excellent hydrogen storage capability. In the above-mentioned laminated material, significant reduction in weight can be realized as compared to the hydrogen storage laminated material described in the above-mentioned Japanese Laid-Open Publication No. 9-59001. Thus, this laminated material can be made highly suitable as a main member of an apparatus intended to be used in, e.g., applications in which rich resources as well as lightness in weight are of great importance. In other words, this laminated material can be made highly suitable as a hydrogen supply source for the hydrogen-utilizing fuel cells, a portable or mobile hydrogen source, or a small hydrogen supply source provided inside and outside the houses, business offices and the like, and thus, can be used as a safe, hydrogen-utilizing power supply or heat source.
The group 2A element or group 3A element included in the first layer of this laminated structure generally has a hexagonal close-packed (hcp) structure at ordinary temperature and pressure, but the above-mentioned laminated structure at least partially includes a bcc structure. The reason why the first layer including the bcc structure has the increased hydrogen storage capacity can be considered as follows: unlike the conventional idea, in the case where the first layer is changed to a crystal of the bcc structure including an element of at least one of the groups 2A and 3A, the number of interstitial sites capable of being occupied by hydrogen atoms is increased to at most nine per atom of the group 2A or 3A element, as shown in FIGS. 4A to 6B. Moreover, since it is possible to control the interatomic distance of the first layer by changing the interatomic distance and constituent element of the second layer, the bonding power between the group 2A or 3A element and hydrogen as well as the size of the hydrogen atom itself can be changed. Thus, hydrogen""s moving speed and moving capability inside the crystal, as well as the bonding power acting on the hydrogen atoms inside the crystal can be adjusted, whereby the number of hydrogen atoms capable of being stored per constituent atom of the first layer can be increased to at most nine. Furthermore, since it is possible to control movement and diffusion of hydrogen, a material capable of easily performing hydrogen absorption and desorption at 100xc2x0 C. or less, and preferably 80xc2x0 C. or less, can be made.
Since the above-mentioned hydrogen storage laminated material of the present invention has such a multi-layer film structure, the hydrogen storage capacity that is significantly superior to that of the conventional bulk hydrogen storage materials can be obtained.
The hydrogen storage laminated material of the present invention can be obtained by laminating two different kinds of substances onto a substrate using, e.g., a vapor phase method like a PVD (Physical Vapor Deposition) method such as vacuum deposition method, ion plating method and sputtering method, and a CVD (Chemical Vapor Deposition) method such as plasma CVD method. In addition to the method as described earlier, in the case where the physical vapor deposition or chemical vapor deposition is conducted in the atmosphere in which high-purity hydrogen gas is present, the bond distance between atoms is increased as compared to the case where hydrogen is not present, whereby the hydrogen storage capability is increased. This is desirably conducted at the hydrogen gas pressure of 1 atm or less, and preferably, in the reduced-pressure hydrogen atmosphere of 0.1 atm or less. Although the effect of the hydrogen gas is not clear, the reason for this can be considered as follows: the hydrogen atoms are taken in simultaneously with formation of the laminated structure, so that the bond distance between the metal atoms resulting from the taking in of the hydrogen atoms is automatically controlled to such a distance that is preferable for taking in and out of hydrogen.
The following consideration is also possible: for example, a change in the electron structure due to increase in the interface (increase in the number of interfaces) or increase in the number of interface atoms resulting from a reduced lamination cycle length of the first and second layers may be involved in the increased hydrogen storage capacity. Therefore, in a preferred aspect of the present invention, the lamination cycle, that is, the length of a unit lamination including the first and second layers is repeatedly laminated.
By thus laminating the lamination structure repeatedly, the hydrogen storage capability can further be improved.
Moreover, in a preferred aspect of the present invention, the second layer is formed from a material having a bulk modulus that is larger than that of the first layer.
By laminating a layer including a group 6A, 7A, 8A element, which has a bcc structure at the ordinary temperature and pressure and also has a larger bulk modulus than a layer including a group 2A or 3A element, and the layer including a group 2A or 3A element, a bcc structure becomes likely to be produced in the first layer. In other words, a metal or the like forming the first layer and having an hop structure at the ordinary temperature and pressure is subjected to elastic deformation at the interface with the second layer due to the high bulk modulus of the second layer, and becomes susceptible to phase transition to the bcc structure at the interface or in the inside of the first layer.
Moreover, in the above-mentioned hydrogen storage laminated material, it is desirable that the laminated material has lattice distortion produced therein.
With the lattice distortion produced in the laminated material, the bcc structure is likely to be produced within the first layer, and particularly at the interface. As a result, the hydrogen storage capability can be improved.
More desirably, in the above-mentioned hydrogen storage laminated material, the first layer includes a group 2A element, Mg, as a main element.
Mg has small specific gravity, and therefore is highly advantageous for reduction in weight. Mg is also rich in resources, and is suitable for industrial mass production. Accordingly, the hydrogen storage laminated material can be used in large quantities in applications in which a reduced weight is important, while maintaining high hydrogen storage capability.
In the above-mentioned hydrogen storage laminated material, it is highly desirable that the second layer includes a group 8A element, Fe, as a main element.
Fe is outstanding as an inexpensive industrial material. The hydrogen storage laminated material using Fe can be mass-produced at low cost, and therefore can be made highly suitable as an electrode material for the nickel-hydrogen secondary batteries, a hydrogen supply source for the hydrogen-utilizing fuel cells, a portable or mobile hydrogen source, or a small hydrogen supply source provided inside and outside the houses, business offices and the like. As a result, this hydrogen storage laminated material can be advantageously used as a new, alternative energy source to the fossil fuel. In particular, combination with a multi-layer material including Mg as a main element in the first layer meets weight and economical requirements, and therefore is highly desirable.